In the past, various techniques have been developed for the purpose of sterilizing or decontaminating biological sludges and wastes. The most common process is the process of mixing lime with the sludge. The reaction of lime with the water in the sludge serves to elevate the temperature of the sludge to a maximum of 100.degree. C.
In the past, various U.S. patents have issued relating to processes for the decontamination and treatment of wastewater sludges. For example, U.S. Pat. No. 4,038,180, issued on Jul. 26, 1977 to N. K. Talbert, describes a process of dewatering sewage sludge in which the sludge from a municipal or industrial sewage treatment facility is mixed with a mineral acid or anhydride thereof to release the entrapped water in the sludge. The resulting mixture of the sludge solids and diluted acid or anhydride is then mixed with a basic material, such as ammonia, such that the heat generated by the reaction of the base and the acid evaporates the water to form either a completely dry mixture of sludge solids and a salt or a mixture having a predetermined moisture content which may be air dried.
U.S. Pat. No. 4,500,428, issued on Feb. 19, 1985 to Lynch et al., describes a method for the treatment of a wastewater sludge using a pair of reaction vessels, sequentially, to treat the sludge. Both of the vessels are pressurized. The first vessel has an aerator for aerating the sludge. This vessel receives sulfuric acid and chlorine therein through a port. A dewatering device is provided upstream of the first vessel. The outlet of the first vessel is coupled to an inlet of the second vessel through another dewatering device. The second vessel creates a final treatment chamber in which the sludge is exposed to ozone, air and lime.
U.S. Pat. Nos. 4,781,842 and 4,902,431, issued to Nicholson, teach processes for the decontaminating of wastewater sludges to a level which meets or exceeds U.S. E.P.A. process standards. The process mixes sludge with an alkaline material sufficient to raise the pH of the end product to 12 or higher for at least one day. This process will raise the temperature to 50.degree. C., but will not sterilize the sludge nor does it eliminate the pathogenic organisms.
U.S. Pat. No. 4,306,978, issued to Wurtz, relates to a process of lime stabilization of wastewater treatment plant sludge. This patent discloses the dewatering of the sludge and intimately mixing calcium oxide to raise the temperature so as to produce a stabilized sludge particle.
U.S. Pat. No. 5,482,528, issued on Jan. 9, 1996 to Angell et al., teaches a pathogenic waste treatment process for the processing of solid waste and for the converting of such solid waste into useful products. This is accomplished by combining the waste with an acid, such as concentrated sulfuric acid, and a base, such as fly ash. These exothermically react and thermally pasteurize the waste and add mineral value to the product. Pozzolanic materials, such as fly ash, agglomerate the product. The calcium oxide in the fly ash reacts with sulfuric acid to form calcium sulfate dihydrate.
None of these prior art patented processes are capable of achieving temperatures, when mixed with the sludge, of greater than 100.degree. C. None of the prior art techniques allow for the shorter drying times as required by 40 C.F.R. Subchapter O, Part 503.32.
U.S. Pat. No. 5,635,069 issued on Jun. 3, 1997 to the present inventors. This patent described a process for treating a waste sludge of biological solids which included the steps of mixing the sludge with an oxide-containing chemical and sulfamic acid so as to elevate the temperature of the sludge, pressurizing the mixed sludge to a pressure of greater than 14.7 p.s.i.a., and discharging the pressurized mixed sludge. The oxide-containing chemical could be either calcium oxide, potassium oxide, or potassium hydroxide. The sludge has a water content of between 5 and 85 percent. The oxide-containing chemical and the acid are reacted with the sludge so as to elevate the temperature of the sludge to between 50.degree. C. and 450.degree. C. The pressurized mixed sludge is flashed across a restricting orifice or passed into a chamber having a lower pressure. The evaporated liquid component can be condensed and used as part of the process or external of the process.
In coal mining processing and handling, enormous tonnages of coal fines are created. Typically, after handling and cleaning are completed, about 15 to 20% of the tonnage mined consists of fines ranging in size from powder to small granules. For the most part, these fines are not directly usable, thereby leaving great quantities of material that is wasted and representing a hazardous and expensive disposal problem. While a portion of the coal fines can be combined with coarser fractions of mine production for sale, the inclusion of all fines often reduces the quality of the product below market requirements. Accordingly, coal fines handling, storage and disposal operations represent a significant and unproductive expense for the industry.
The fines material from mining operations is frequently in the form of a wet filter cake containing between about 20 and 30% moisture, depending upon its size distribution and ash content. In a dry state, the fines are generally predominantly passable through a 28-mesh screen, a size that may be used for pelletizing and/or briquetting purposes. As used in this disclosure, the terms "pellet", "briquet", "log" and "block" are used interchangeably and are intended to refer to all forms of pellets, briquets, logs, blocks and other coal agglomerates produced by binding coal fines into a concrete material. Similarly, the terms "pelletizing" and "briquetting" are intended to refer to equivalent processes for producing coal agglomerates that are also used interchangeably.
In the past, numerous processes have been proposed and implemented for pelletizing and/or briquetting coal fines. For example, U.S. Pat. No. 444,994, issued over a century ago, teaches that coal dust can be pelletized by saturating it with a saturation of starch, pressing or otherwise forming it into blocks or lumps and drying it in the sun or by other suitable means. U.S. Pat. No. 852,025 discloses preparing coal for briquetting by drying and heating it, mixing in an asphaltic binder material, then heating, cooling and compacting the mixture. U.S. Pat. No. 1,121,325 discloses briquetting coal fines by mixing dry coal and starch, adding steam that is saturated with oil, then compressing and thermally drying the mixture. U.S. Pat. No. 1,851,689 discloses briquetting coal fines by mixing the coals with a starch/oil emulsion and then autoclaving it at 300.degree. F. U.S. Pat. No. 4,049,392 discloses an extrusion apparatus for extruding rod-like bodies from coal-containing particulate mixtures, and having means for adjusting the length and density of the extruded particles.
Many natural and synthetic polymers have been used as binders for coal fines. U.S. Pat. No. 5,244,473 teaches that a binder for coal fines can be made from a phenol-aldehyde resin mixed with a polyisocyanite in the presence of a catalyst. U.S. Pat. No. 5,089,540 teaches that a binder for foundry molds can be an ester-cured alkaline phenolic resin which can be enhanced by conditioning the retained sand with a solution containing an amine and a silane. U.S. Pat. No. 5,009,671 teaches that coal briquets can be made by using a starch binder mixed with molasses and water. U.S. Pat. No. 4,862,485 teaches how to make coal pellets by mixing coal particles with polyvinyl alcohol, calcium oxide, and/or magnesium oxide and water. U.S. Pat. No. 4,738,685 teaches how to cold press coal fines with molasses, an inorganic hardening agent such as calcium carbonate, calcium phosphate, iron oxide, aluminum oxide and optionally with an acid. U.S. Pat. No. 4,618,347 teaches how to make coal pellets from coal fines and a binder consisting of lignosulfanate plus sodium dichromate, or using sulfuric acid as a curing agent. U.S. Pat. No. 4,586,936 shows how to make coal pellets from lower rank coal mixed with cationic polyurethane and polyvinyl alcohol. U.S. Pat. No. 4,169,711 teaches that coal particles mixed with coal fines can be briquetted into "smokeless" fuel logs when mixed with sodium silicate and potassium silicate. Finally, U.S. Pat. No. 3,966,427 teaches how to make coal briquets using bitumen or bitumen emulsions as binders.
Many prior art binders use water in the process of mxing with coal fines to produce briquets, thereby further increasing the moisture content of the product. Thus, for example, when starch-based binders are used, the resulting green pellets must be dried to achieve acceptable fuel performance and reduced transportation costs. In addition, prior art binders are derived from useful and often expensive raw materials, such as natural and synthetic polymers; therefore, they add significantly to the overall cost of the briquet.
Recently, various U.S. patents have issued on processes for improving the abililty to utilize such coal fines. U.S. Pat. No. 5,916,826, issued on Jun. 29, 1999, to D. H. White, describes a coal agglomerate produced by the combination of coal fines with a binder material obtained from the direct liquefaction of biomass material. The direct liquefaction is carried out in the absence of oxygen at temperatures of between 450.degree. F. and 700.degree. F. and typical pressures between 200 and 3,000 p.s.i. The liquified bio-binder base is mixed with additives such as fast pyrolysis tars and petroleum asphalt and the resulting mixture is sprayed on coal fines preheated to at least 250.degree. F. and allowed to react at between 300.degree. F. and 400.degree. F. The resulting mixed mass is then pelletized by the application of pressure in conventional equipment.
U.S. Pat. No. 5,743,924, issued on Apr. 28, 1998, to Dospoy et al., describes a pelletized fuel composition comprising coal fines in an amount of between 60 and 80% by weight, paper making sludge in an amount of between 10 to 30% by weight and a shredded polymeric material in an amount of between 0.5% to 15% by weight. The pelletized coal fines have the necessary degree of hardness and structural integrity without the necessity of a binder.
U.S. Pat. No. 5,599,361, issued on Feb. 4, 1997, to G. W. Ford, Jr., teaches a method for forming a solid fuel product comprising carbon fines bound together chemically. In particular, methyl ethyl ketone into which styrene polymer resin is dissolved is added to the wet coal fines. A polyvinyl acetate and water emulsion is added to the coal fines. The resulting mixtures are pressed into briquets using an injection pressure of 3,000 p.s.i.
In each of the aforementioned processes, the addition of chemicals, binders and cementing agents adds to the cost of producing the resulting pellet or briquet. As such, the cost of forming such briquets is at such a level that the briquets are not actually used as a fuel product.
It was found that the end product of the BIOSET (TM) process (identified in U.S. Pat. No. 5,635,069 to the present inventors) included lime, silicates and aluminum. As such, this treated sewage sludge can serve as a cement binder. Calcium hydroxide (lime), alum (clay) and sand will create a pozzolanic reaction. As such, the sewage sludge resulting from the BIOSET (TM) process can be mixed with the coal fines so as to produce a high BTU fuel product. Since the treated sewage sludge of the BIOSET (TM) process is pathogen-free and vector-free, it can be used in commercial processes. Furthermore, the organic material within the treated sewage sludge adds to the BTU content (approximately 1200 BTU per pound).
It is an object of the present invention to provide a process which makes valuable use of coal fines.
It is another object of the present invention to provide a process which produces a high BTU fuel product.
It is a further object of the present invention to provide a process which utilizes treated sewage sludge to bind coal fines together.
It is a further object of the present invention to provide a fuel product which qualifies for the alternative fuel tax credit.
It is still another object of the present invention to provide a process which renders sewage sludge vector-free, pathogen-free and sellable.
It is still another object of the present invention to provide a process whereby coal fines can be made easily transportable.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.